Method for manufacturing a passive integrated matching network for power amplifiers

ABSTRACT

An impedance matching network is integrated on a first die and coupled to a second die, with the first and second dies mounted on a conductive back plate. The impedance matching network comprises a first inductor bridging between the first and second dies, a second inductor coupled to the first inductor and disposed on the first die, and a metal-insulator-metal (MIM) capacitor disposed on the first die. The MIM capacitor has a first metal layer coupled to the second inductor, and a second metal layer grounded to the conductive back plate. A method for manufacturing the integrated impedance matching network comprises the steps of forming an inductor on a die, forming a capacitor on the die, coupling the capacitor to the inductor, coupling the die bottom surface and the capacitor to a conductive plate, and coupling the inductor to another inductor that bridges between the die and another die.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 11/179,685 filed on Jul.11, 2005.

TECHNICAL FIELD

The present invention relates generally to electronic components thatreceive and transmit a signal. More particularly, the present inventionrelates to matching networks for use with such electronic components.

BACKGROUND

Electronic devices and components that are designed for high frequencydata communication applications have numerous applications. One commonpractical application for such devices and components is cellulartelephony systems. In this regard, the need for component integrationhas become progressively more important for increasingly miniaturizedhigh performance cellular phones with advanced features.

Cellular phone high power amplifier stations use matching circuits andpower amplification circuits to match the impedances in a relativelyweak cellular telephone signal and to amplify and transmit the signal.The LDMOS power amplification system illustrated in FIG. 1 is onein-package power amplification system that successfully incorporatespassive elements, including passive inductors and capacitors, in ahighly integrated impedance matching network. The illustrated poweramplification system 100 includes a metal flange 102 that functions as asupport and as an electrical ground. A ceramic substrate 104 is formedon the metal flange 102 and insulates the flange 102 from the poweramplification circuitry, which includes a gate lead 106, a drain lead108, and an array of inductor bond wires 114 that connect matchingcapacitors 110 and active devices 1 12.

FIG. 2 is a circuit diagram for a power amplifier (PA) system. Theequivalent circuit schematics for the assembly represented in FIG. 1 isshown in box 200. Box 210 represents a large capacitance in series witha small inductance, which together realize the shunt LC matchingnetwork. This part of the MOSCAP matching network includes long bondwires that connect the MOSCAP die to the LDMOS output drain, which islocated on a base station power amplifier die. The bond wires arecommonly assembled with a height of approximately 45 mils (˜1.1 mm) inorder to realize an appropriate inductance value. The bond wire lengthfor this assembly creates difficulties when assembling and operating thepower amplifier. Slack bond wire is difficult to control duringmanufacturing. Further, longer bond wires become hotter and lessreliable during operation than shorter bond wires, and also causerelatively wide inductance variations.

One of the most significant parameters for evaluating the performance ofa shunt matching network is the network's Q factor. The Q factor refersto the measure of quality of a particular frequency response, and iscorrelated with a storage:loss ratio. The simulated Q factor for thecurrent bond wire+MOSCAP system is about 60, although 60 is toward thelower end of the range of acceptable Q factors for the shunt LC matchingnetwork.

Accordingly, it is desirable to provide a shunt LC matching networkhaving a Q factor that is higher than 60 and that can be manufacturedwith efficiency. It is also desirable for the matching network toutilize quality passive devices in order for the matching network to beeasily controlled during manufacturing, and to have a relatively lowparts count. It is also desirable for such a matching network to haveimproved reliability. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIG. 1 is a top isometric view of an LDMOS power amplification system;

FIG. 2 is a circuit diagram for a LDMOS power amplifier system includinga shunt LC output impedance matching network;

FIG. 3 is a side view of an exemplary LC output impedance matchingshunt, including an integrated passive inductor and capacitor in series,according to an embodiment of the present invention;

FIG. 4 is a side view of a conventional shunt LC output impedancematching network;

FIG. 5 is a top isometric view of the LC output impedance matching shuntillustrated in FIG. 3; and

FIG. 6 is a flow diagram outlining an exemplary method for manufacturingthe shunt LC output impedance matching shunt according to an embodimentof the present invention.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the invention or the application and uses ofthe invention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

The invention may be described herein in terms of functional and/orschematic components. It should be appreciated that such components maybe realized in any number of practical ways. For example, an embodimentof the invention may employ various elements, e.g., conductive traces,wire bonds, integrated passive devices, semiconductor substratematerials, dielectric materials, or the like, which may havecharacteristics or properties known to those skilled in the art. Inaddition, those skilled in the art will appreciate that the presentinvention may be practiced in conjunction with any number of practicalcircuit topologies and applications, and that the circuits describedherein in conjunction with the inventive impedance matching circuits aremerely example applications for the invention.

According to one embodiment of the invention, a method is provided formanufacturing an integrated impedance matching network. The methodcomprises the first step of forming a first inductor on a first diehaving top and bottom surfaces. The first die may be a die thatcomprises GaAs, and may have a thickness that is between about 75 andabout 200 μm. The method further comprises the steps of forming acapacitor first metal layer on the first die top surface, forming aninsulator layer on the capacitor first metal layer, forming a capacitorsecond metal layer on the insulator layer, and forming a third metallayer to couple the capacitor second metal layer to the first inductor,mounting the first die bottom surface to a conductive plate, couplingthe capacitor first metal layer to the conductive plate, and couplingthe first inductor to a second inductor that bridges between the firstdie and a second die that is mounted on the conductive plate.

The second die may comprise a power amplification circuit. Moreparticularly, the integrated impedance matching network may be part of apower amplifier for a base station that amplifies and transmits remotesignals.

The second inductor may comprise a plurality of bond wires, and thefirst inductor may comprise a plurality of metal strips. Each of themetal strips may be more than 8 μm in thickness, and more than 40 μm inwidth. Further, each of the bond wires may be between about 10 mils andabout 30 mils in height (between about 250 and about 750 μm).

The step of coupling the capacitor first metal layer to the conductiveplate may comprise the steps of forming a via hole through the first diebefore mounting the first die bottom surface, and filling the via holewith an interconnect metal.

The first inductor, and the capacitor first and second metal layers maybe formed from the same materials. Further, the step of forming thefirst inductor may be performed while performing one or more of thesteps of forming the capacitor first and second metal layers and thestep of coupling the capacitor second metal layer to the first inductor.If one or more of these steps are performed substantiallysimultaneously, the first inductor is formed using the same metals asthe capacitor first and second metal layers and/or the material used tocouple the second metal layer to the first inductor.

According to another embodiment of the invention, an impedance matchingnetwork is integrated on a first die and coupled to a second die, withthe first and second dies mounted on a conductive back plate. Theimpedance matching network comprises a first inductor bridging betweenthe first and second dies, a second inductor coupled to the firstinductor and disposed on the first die, and a metal-insulator-metal(MIM) capacitor disposed on the first die. The MIM capacitor has a firstmetal layer coupled to the second inductor, and a second metal layergrounded to the conductive back plate. The first die may be a diecomprising GaAs, and may have a thickness that is between about 75 andabout 200 μm. The second die may comprise a power amplification circuit.

The first inductor may comprise a plurality of bond wires, and thesecond inductor may accordingly comprise a plurality of metal strips.Each of the metal strips may be more than 8 μm in thickness, and morethan 40 μm in width. Further, each of the bond wires may be betweenabout 10 mils and about 30 mils in height. The second inductor, and themetal-insulator-metal capacitor first and second metal layers may beformed from the same materials.

The first die may have a via hole formed therethrough between theconductive back plate and the metal-insulator-metal capacitor secondmetal layer. In such an embodiment, the via hole is filled with aninterconnect metal.

As discussed in the preceding background description, the conventionalshunt LC matching network illustrated in FIG. 1 utilizes long bond wiresto connect a MOSCAP die with a base station power amplifier LDMOS die,and to provide inductance in series with a high capacitance. The presentinvention includes an integrated passive device (IPD) design for theshunt LC matching network that optimizes the bond wire length by usingone or more parallel micro-striplines to provide inductance in place ofbond wires. In other words, the IPD is electrically connected to theLDMOS die using relatively small bond wires, and the shunt LC inductanceis distributed between both the bond wires and the IPD micro-striplineinductors. The micro-stripline inductance is more easily reproduced thanlong bond wire inductance. Further, because a significant amount ofinductance is attributed to the IPD die, the total inductance is tightlycontrolled. The micro-stripline inductors have better thermaldissipation properties than do long bond wires. Consequently, the IPDdesign provides improved thermal properties and reliability.

FIGS. 3 and 5 are, respectively, side and top isometric views of anexemplary IPD design 10. The LC impedance output matching shunt isdisposed on a metal platform 113, which also functions as a ground. Abond wire 12 is used to connect two separate dies disposed on theplatform 113. Situated on one platform area is the previously discussedbase station power amplifier LDMOS die 116, and an IPD die 22 issituated on a separate platform area.

Exemplary bond wires 12 include coils that create inductance between theLDMOS die 116 and the IPD die 22. FIG. 4 is a side view of aconventional assembly 220, including a bond wire 122 that createsinductance between an LDMOS die 116 and a MOSCAP die 124. Contactbetween the conventional bond wire 122 and the dies 116, 124 isfacilitated by bond pads 118, 120. As seen when comparing the assembliesof FIGS. 3 and 4, the bond wires 12 of the present invention is muchlower in height, and therefore shorter in length, than the conventionalbond wires 122. According to one embodiment, the bond wires 12 of thepresent invention are between about 10 mils and about 30 mils in height(between about 250 and about 750 μm), while the conventional bond wires122 have a height of about 45 mils (˜1.1 mm).

The assembly for the base station power amplifier LDMOS die 116 isessentially the same for both the conventional assembly 220 and theexemplary assembly 10, although in the current invention the LDMOS die116 may alternatively be a III-V compound semiconductor die. However,the MOSCAP die 124 in the conventional LC shunt is replaced with the IPDdie 22, which includes a plurality of integrated inductor strips 14coupled to the bond wires 12, and an integrated MIM capacitor 26 thatincludes lower and upper metal layers 16, 20 and a dielectric layer 18between the metal layers 16, 20. The MIM capacitor lower metal layer 20is grounded to the metal platform 113 by way of a via hole 24 formedthrough the IPD die 22.

The IPD die 22 is a substrate having passive components integrallyformed therewith. The substrate 22 may be a high resistivity siliconwafer or other insulating or semi-insulating wafer. An exemplarysubstrate is a GaAs wafer because GaAs is an optimal material forlow-loss RF passive elements. A suitable dielectric such as SiN isdeposited onto the substrate 22, although the dielectric layer is notillustrated.

The inductor strips 14 are coupled to the bond wires 12, and thecombined bond wires 12 and inductor strips 14 together provide acombined inductance in series with the MIM capacitor 26. For ease offabrication the IPD metals, including the inductor strips 14, and theMIM capacitor lower and upper metal layers 16, 20 are formed from thesame materials such as gold, copper, or aluminum in an exemplaryembodiment, although a variety of conductive metals may be used. Becausethe IPD inductor strips 14 place a significant amount of inductance onthe IPD die 22, the total inductance is tightly controlled and highlyreproducible. Further, the inductor strips 14 have improved thermaldissipation properties with respect to bond wires.

The dimensions for the inductor strips 14 are pre-calculated withinductor loss and inductor resonance as major factors. For example,inductor loss improves with wider inductor strips 14. However, theinductor resonance, which is determined by the inductor parasiticcapacitance to ground, decreases as the inductor strips 14 are widened.Other factors that affect inductor loss include the inductor thickness,and spacing to ground. Exemplary inductor strips 14 are more than 8 μmin thickness, more than 40 μm in width, and between about 10 mils andabout 30 mils in height (between about 250 and about 750 μm).

The distance between the inductor strips 14 and the metal platform 113that functions as a ground is determined by the IPD die thickness.Factors that affect the optimal IPD die thickness include improving theinductor loss, and ease of manufacture. For example, a thick IPD dieimproves inductor loss. However, subsequent die processing, as information of the via hole 24 through the IPD die 22, is less efficientfor a relatively thick IPD die 22. In the exemplary embodiment employinga GaAs substrate, the IPD die 22 is between about 3 and about 8 mils(between about 75 and about 200 μm) in thickness, and the inductorstrips 14 are consequently disposed between about 75 and about 200 μmfrom the metal platform 113.

The inductor strips 14 are connected to the MIM capacitor upper metallayer 20 using a plurality of air bridges 28 disposed over the IPD die22. In a preferred embodiment, the air bridges 28 are formed usinglithography, deposition, and plating techniques. The air bridges 28 mayalso be formed using bond wires, although such an assembly would be lessefficient and relatively bulky. Similarly, the metal filling the viahole 24 and extending through the IPD die 22 to connect the MIMcapacitor lower metal layer 16 to the metal platform 113 is preferablyformed using lithography, deposition, and plating techniques instead offrom bond wires to reduce cost, substrate size, and overall packagesize.

FIG. 6 is a flow chart that outlines the steps for manufacturing anexemplary IPD die and coupling it with bond wire coils to produce ashunt LC matching network. In order to form the IPD components,including the metal inductor strips 14 and the capacitor layers 16, 20,the metal layers are deposited and the desired conductive traces areformed by sputtering, plating, evaporation, and/or etching or othersuitable methods. The metal layers are typically referred to as “metal1,” “metal 2,” “metal 3,” and so on to indicate the order in which theyare deposited or formed onto the substrate during the fabricationprocess. Formation of each of the metal layers may include a series ofsteps including depositing a seed metal by a suitable method such assputtering or evaporation, applying a photoresist, covering the exposedseed metal with a conductive metal plating, and removing the photoresistand non-plated seed material. Although a variety of metals can be usedto form the IPD components, in the following exemplary procedure theseed metal is TiW and the plating metal is Au.

Step 30 comprises formation of a metal 1 layer on a GaAs wafer that willlater be processed to a suitable shape to form the IPD die 22. The wafercan also be a high resistivity silicon or other suitable insulating orsemi-insulating material. At least the wafer areas on which the metal 1layer is to be formed are covered with an insulating material such asSiN. The entire metal 1 layer, including the seed layer and theconductive metal plating, forms the MIM capacitor lower metal layer 16,and has a thickness of about 0.5 to about 3

Step 32 comprises forming the capacitor dielectric layer 18 over themetal 1 layer. The dielectric layer 18 may be formed from any suitabledielectric material, and in the present example is SiN. A series ofdeposition and etching steps are used to form the dielectric layer 18.One suitable deposition method includes a plasma-enhanced chemical vapordeposition process.

Step 34 comprises forming a metal 2 layer over the capacitor dielectriclayer 18. The metal 2 layer forms the MIM capacitor upper layer 20, andhas a thickness of about 1 μm to about 3 μm.

Step 36 comprises forming a metal 3 layer to form at least the airbridge 28. The air bridge 28 connects the inductor strips 14 to the MIMcapacitor upper layer 20, and has a thickness of about 7 μm to about 13μm. The inductor strips 14 are formed during one or more of the metal 1,metal 2, and metal 3 formation steps 30, 34, and 36, particularly wheneach of the metal layers are formed from the same metal or compatiblemetals for forming the inductor strips 14.

After forming the inductor strips 14 and the MIM capacitor 26 on thewafer during steps 30 to 36, a passivation layer is formed over theseIPD components as step 38. A suitable passivation layer is SiN, althoughmany other suitable materials may be used to protect the components onthe wafer.

Step 40 comprises forming the via hole 24 through the wafer. The viahole 24 connects the previously-formed MIM capacitor lower layer 16 tothe metal platform 113, and the MIM capacitor lower layer 16 is formedover the wafer area through which the via hole 24 is to be formed.Consequently, the via hole 24 is formed by mounting the wafer face-downon a carrier, lapping and polishing the wafer to a final thickness ofabout 3 to about 8 mils (about 75 to about 200 μm), and using aphotolithography process to dry or wet etch through the wafer until thevia hole 24 reaches the MIM capacitor lower layer 16.

Step 42 comprises filling the via hole 24 with a suitable interconnectmetal 25. In an exemplary embodiment, the interconnect 25 is formed fromthe same metal as the MIM capacitor lower layer 16, although anysuitable conductive metal may be used to form the interconnect 25.

Steps 30 to 42 effectively complete formation of the wafer, with theexception of shaping and attachment steps. Step 44 comprises processingthe wafer to complete formation of the IPD die 22. The wafer is mountedto a backing using saw tape, and the wafer is sawed to a predeterminedsize and shape. Then, the completed IPD die 22 is attached to the metalplatform 113 by performing a soldering or other joining technique. Inthis configuration, the interconnect 25 is joined to the metal platform113. Finally, step 46 comprises attaching the bond wires 12 to theinductor strips 14 using a wire bonding or other joining technique, andthereby completing the LC shunt matching circuit.

The previously-described LC shunt matching network improves matchingimpedance reproducibility by partially substituting thin film-basedprocessing techniques for conventional wire-bonding techniques withoutsacrificing the amount of inductance that wire bonds provide. Forassemblies in which conventional bond wire inductors would be too tallor bulky to be packaged, the integrated passive LC shunt provides apackage-integrated solution. Further, the shortened bond wire inductorsimprove thermal performance and system reliability. In addition, theintegrated passive LC shunt improves the matching network Q factor andthereby improves the overall performance of the power amplificationsystem, or other system, in which the circuitry is incorporated.

While at least one example embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexample embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the invention in anyway. Rather, the foregoing detailed description will provide thoseskilled in the art with a convenient road map for implementing thedescribed embodiment or embodiments. It should be understood thatvarious changes can be made in the function and arrangement of elementswithout departing from the scope of the invention as set forth in theappended claims and the legal equivalents thereof.

1. An impedance matching network integrated on a first die and coupled to a second die, the first and second dies being mounted on a conductive back plate, the impedance matching network comprising: a first inductor bridging between the first and second dies; a second inductor coupled to the first inductor and disposed on the first die; and a metal-insulator-metal capacitor disposed on the first die, and having a first metal layer coupled to the second inductor, and a second metal layer grounded to the conductive back plate.
 2. The impedance matching network according to claim 1, wherein the first die comprises GaAs.
 3. The impedance matching network according to claim 1, wherein the first die is between about 75 and about 200 μm in thickness.
 4. The impedance matching network according to claim 1, wherein the first inductor comprises a plurality of bond wires, and the second inductor comprises a plurality of metal strips.
 5. The impedance matching network according to claim 4, wherein each of the metal strips is more than 8 μm in thickness, and more than 40 μm in width.
 6. The impedance matching network according to claim 4, wherein each of the bond wires is between about 10 mils and about 30 mils in height.
 7. The impedance matching network according to claim 1, wherein the second die is selected from the group consisting of an LDMOS and a III-V compound semiconductor die.
 8. The impedance matching network according to claim 1, wherein the second die comprises a power amplification circuit.
 9. The impedance matching network according to claim 1, wherein the first die has a via hole formed therethrough between the conductive back plate and the metal-insulator-metal capacitor second metal layer, the via hole filled with an interconnect metal.
 10. The impedance matching network according to claim 1, wherein the second inductor is coupled to the metal-insulator-metal capacitor first metal layer using a third metal, and wherein the second inductor, the metal-insulator-metal capacitor first and second metal layers, and the third metal are formed from the same materials.
 11. A power amplifier for a base station that amplifies and transmits remote signals, the power amplifier comprising: a conductive plate; a first die comprising a power amplification circuit and mounted on the conductive plate; a second die mounted on the conductive plate and having an impedance matching network integrated thereon and coupled to the first die, the impedance matching network comprising: a first inductor bridging between the first and second dies; a second inductor coupled to the first inductor and disposed on the second die; and a metal-insulator-metal capacitor disposed on the second die, and having a first metal layer coupled to the second inductor, and a second metal layer grounded to the conductive plate.
 12. The power amplifier according to claim 11, wherein the second die comprises GaAs.
 13. The power amplifier according to claim 11, wherein the second die is between about 75 and about 200 μm in thickness.
 14. The power amplifier according to claim 11, wherein the first inductor comprises a plurality of bond wires, and the second inductor comprises a plurality of metal strips.
 15. The power amplifier according to claim 14, wherein each of the metal strips is more than 8 μm in thickness, and more than 40 μm in width.
 16. The power amplifier according to claim 14, wherein each of the bond wires is between about 10 mils and about 30 mils in height.
 17. The power amplifier according to claim 11, wherein the first die is selected from the group consisting of an LDMOS and a III-V compound semiconductor die.
 18. The power amplifier according to claim 11, wherein the second die has a via hole formed therethrough between the conductive plate and the metal-insulator-metal capacitor second metal layer, the via hole filled with an interconnect metal.
 19. The power amplifier according to claim 11, wherein the second inductor, and the metal-insulator-metal capacitor first and second metal layers are formed from the same materials.
 20. The power amplifier according to claim 11, wherein the second die comprises an insulating or semi-insulating substrate. 